Hydraulic tensioners have been widely used to maintain proper tension in timing chains and timing belts, which transmit rotation between the crankshaft and one or more camshafts in a vehicle engine. These tensioners also prevent vibration generated by travel of a transmission chain or belt.
As shown in FIG. 7, a conventional hydraulic tensioner 500 comprises a plunger 520 slidably protruding from a plunger accommodating hole 511 formed in a housing 510. A plunger-biasing spring 530 is accommodated in a high pressure oil chamber R, formed between the plunger-accommodating hole 511 and the plunger 520, to bias the plunger 520 in the protruding direction. A check valve unit 540, incorporated in a bottom portion of the plunger-accommodating hole 511, blocks reverse flow of oil out of the high pressure oil chamber R. The check valve unit 540 comprises a check ball 541, a ball seat 544 which faces the check ball 541, a conical coil spring 545, which biases the check ball 541 onto the ball seat 544, and a retainer 542, which supports the conical coil spring 545 and limits the movement of the check ball 541.
As shown in FIG. 8, a clearance, D1-d1, is provided between the check ball 541 and the retainer 542 to ensure the proper rate of inflow of oil for the maximum area of the opening obtained when the check valve is opened. (See, for example, Japanese Laid-open Patent Publication No. 2003-247616, Page 1, FIG. 1.) The check ball valve 541 moves irregularly in this clearance, and consequently, a violent motion of the check ball 541 is generated. As a result of this irregular and violent motion of the check ball, the hydraulic tensioner cannot adequately follow changes in tension, especially when the engine is operating at high rotational speeds. Adequate follow-up properties are particularly difficult to achieve especially when the hydraulic tensioner is disposed laterally on the engine, since, when the direction of flow through the check valve is not vertically upward, for example when the direction of flow is horizontal, gravity displaces the check ball from alignment with its seat, further contributing to the irregular and violent movement of the check ball.
The clearance between the ball and the inner wall of the ball guide should be sufficient to ensure that the inflow rate of oil corresponds to the maximum area of the opening between the check ball 541 and seat 544, so that the ball does not restrict oil flow when the check valve is fully opened. However, the above-mentioned clearance is influenced by the stroke of the check ball. That is, when the stroke of the check ball is increased, the maximum area of the opening between the check ball and the retainer must be increased in order to ensure adequate flow of oil. As a result, in the case of a check valve having a relatively long stroke, the necessary clearance between the ball and the inner wall of the ball guide is larger than in the case of a check valve having a relatively short stroke. Accordingly, in the conventional hydraulic tensioner the violent motion of the check ball could not be suppressed.
Objects of the invention are to solve the above-mentioned problems of the prior art, and to provide a hydraulic tensioner which can suppress violent motion of the check ball in its check valve unit, and accurately follow changes in tension, especially at high rotational speeds, irrespective of the stroke of the check valve and the arrangement of the tensioner.